s have hitherto been
studied in mountainous countries, their presence has been supposed to
imply the presence of mountains, this impression being strengthened by
the downward and onward movement of existing glaciers, so long supposed
to be exclusively due to the slopes along which all modern glaciers
advance. Were it true that glaciers move solely or mainly on account of
the sloping bottom on which they rest, and that they can advance only
on an inclined plane, all the phenomena concerning drift, polished and
furrowed surfaces, boulders, etc., in America, would hardly justify us
in assuming a moving sheet of ice as their cause. But we have seen that
the phenomena of glaciers, like those of currents, are in great part
meteorological. The Gulf-Stream does not flow toward the English shore
because the ocean-bottom slopes eastward; nor does the cold current of
Baffin's Bay run down-hill when it pours its icy waters southward upon
our northeast coast. Their course is determined by laws of temperature,
and so have we also seen that the motion of glaciers is mainly
determined by conditions of temperature, although, in this case, an
internal mechanical action is combined with external influences; and
while it is true that glaciers, as they now exist, are dependent upon
the shape of the valleys in lofty mountain-chains, yet under different
geographical conditions the same phenomena may be produced over level,
open countries.

I believe that circumstances similar to those determining the more rapid
advance of the glaciers from higher to lower levels at that point where
the alternate thawing and freezing, the infiltration of water and
consequent expansion of the ice under frost, are greatest, would also
determine the motion of a large body of ice from north to south, since
it would be along its southern limits that these conditions would
prevail; while the great reservoir of snow at the north would correspond
to the upper troughs of the present glaciers, from which their lower
ranges are constantly fed. The change of snow into ice is owing to
alterations of temperature, to partial melting and subsequent freezing,
constantly renewed,--and also to the sinking of the mass upon itself in
consequence of its own weight, the lower portions being thus forced out
by the pressure of the superincumbent ice. Upon an inclined plane the
movement consequent upon these changes will of course be downward; but
what would be the result, if a field o